Hammersmith Flyover carries the A4 trunk road and was built to cope with increasing traffic in central London and travelling to Heathrow Airport. It was the first UK structure to use precast segmental post-tensioned concrete, and because London County Council saved money over a conventional structure they paid Maunsell & Partners an ex-gratia fee for the design. Recent strengthening work included retro-fitting additional post-tensioning, and using ultra-high performance fibre reinforced concrete anchorages — a UK first.

The need for an additional traffic route through Hammersmith was recognised in the late 19th century but its alignment was not finalised until 1935. In 1948 there was further discussion over the relative merits of a tunnel or viaduct.

The Ministry of Transport decided to award a grant towards the cost of a flyover in 1956, perhaps influenced by the constraints of maintaining traffic flow and the cost of demolishing property for road widening. New dual carriageways at the west end of the proposed viaduct (known as the Cromwell Road extension, completed 1956) had doubled the number of vehicles using the route between July 1956 and July 1960.

Hammersmith Flyover overflies part of the Great West Road, Fulham Palace Road, London Underground (District and Piccadilly lines), Talgarth Road and three other roads. The contract also included widening the existing railway overbridge, modifying existing services and constructing a pedestrian subway at Fulham Palace Road.

The engineering designers Guy Anson Maunsell (1884-1961) and Charles Peter Wroth (1929-91) chose to use precast reinforced concrete elements wherever possible, combined with the then unusual technique of post-tensioning with steel tendons. Being able to precast most of the units offsite (at Heston 14km away) was a huge advantage in a site so congested and cramped, especially as traffic had to flow unimpeded throughout the construction period.

Construction began at the west end and proceeded eastwards. Erection of precast units and post-tensioning were carried out during off-peak hours, but all other operations continued in live traffic. On site working space was limited to just an 8.5m wide strip. The beam and cantilever units were placed by a steel gantry (designed by F.S. Jackson for the contractor) driven by an electric motor that ran on rails over the beam units already in position. One span could be completed every three weeks.

The flyover is supported on a single central row of columns. It has 16 spans — 11 at 42.7m long, two of 36.6m, two of 28.8m and one of 22.6m, giving a suspended length of 622.7m. The approach ramps at either end increase the total length to 862.9m. There is only one expansion joint, 64m west of the flyover's halfway point, which can accommodate movement of up to 360mm.

The deck is 18.6m wide overall, with a 7.3m wide carriageway and a service footway on either side of the 1.5m wide median strip. The carriageways have a crossfall of 1 in 40 towards the median strip, which carries the lamp standards and access manholes. The deck has a minimum clearance of 5m over all the road crossings.

The main structural element is a continuous hollow spine beam 7.9m wide, which varies in depth from 2m at midspan to 2.75m at the supports. It has curved undersides and was precast in units 2.6m long alternating with 300mm thick transverse cantilever units, with fine concrete joints 76mm thick between. There are 202 units each of beams and cantilevers.

These units are post-tensioned together longitudinally with 29mm diameter steel cables, using the Gifford-Udall system and post-tensioning each span individually on completion. There are four clusters of 16 tendons, one cluster on either side of each of the two inner spines — passing through 250mm diameter ducts within the lower flange midspan. The central compartment of the beam carries water mains, drainage pipes and electric cables for street lighting and road heating.

The 18m wide heavily reinforced concrete cantilever units act as diaphragms for the spine beams as well as supporting the 200mm thick precast reinforced concrete slabs 4.4m wide and 2.9m long that form the outlying parts of the carriageways. The slabs are joined with in situ concrete and the roadway was waterproofed with a 19mm layer of mastic asphalt. Continuous in situ reinforced concrete edge beams 1.1m wide and 0.45m deep, flanked by safety fencing, form the footways.

The tapering concrete columns were cast in situ in a single pour and are connected to the spine beam by vertical prestressing bars. They have a hollow rectangular section that varies from 3m by 2.6m at the top to 2.6m by 2.1m at the bottom with walls 600mm thick and 4.9m tall. Each column has 24 Lee-McCall high tensile steel tendons 32mm in diameter, prestressed from the top and passing through ducts inside the column walls with anchorages cast into the column bases.

A pair of steel roller bearings each 200mm in diameter and 800mm long at the base of every column accommodates any longitudinal movement in the flyover. The columns are founded on reinforced concrete footings bearing on the Thames ballast (sand and gravel) underlying the site. Foundation depth varies, with those adjacent to the railway cutting being taken to the same level as its mass concrete retaining walls.

The approach ramps comprise compacted fill between reinforced concrete retaining walls, with reinforced concrete roadway slabs laid over the fill. The ramps have a gradient of 1 in 20 and join the abutments some 4.6m above ground level. The abutments are cellular reinforced concrete structures that anchor the ends of the flyover and prevent longitudinal movement.

The flyover was completed in 1962 and cost about £1.2m. In the mid-1960s, its electric deck heating system was disconnected owing to rising energy prices. The cabling was removed. Road salt was then used for de-icing, a measure not envisaged in the original design.

In 1999-2000, the Highways Agency found corrosion in the post-tensioning tendons caused by water leaking from the deck. In 2000, Transport for London took ownership of the structure and embarked on remedial works. The original waterproofing was replaced with new mastic asphalt in August 2003.

In 2009, inspections by Enterprise Mouchel inside the tendon ducts showed significant deterioration. Structural monitoring commenced with 400 acoustic sensors plus inclinometers and strain gauges on the flyover’s eastern section. They were detecting one wire break a day — urgent repairs were required.

Further investigation in 2011 revealed extensive voids in the grout surrounding the tendons and active corrosion of the tendon wires. The flyover closed on 22nd December 2011, and was later subject to a 7.5 tonne weight limit. In January 2012, emergency strengthening (phase 1) was undertaken, installing remedial prestressing tendons over the piers, costing £17m. It re-opened fully on 28th May 2012, just in time for the London 2012 Olympic and Paralympic Games.

More-complex strengthening (phase 2), costing around £100m, commenced in October 2013 (completed September 2015). Designed by Ramboll and Parsons Brinckerhoff in a joint venture, the works are intended to avoid the need for major maintenance for at least 60 years. Despite tight working conditions, minimum headroom clearance was maintained and the viaduct remained open in daylight hours during construction.

The new below-deck post-tensioning system had to be independent of the original prestressing, which had to stay, and installed without compromising the structural integrity of the flyover. It is believed to be the first time a viaduct has received a full retro-fitted replacement system without removal of the original.

The placement of the new tendons was critical. Extensive 3D modelling and laser scanning were used, leading to a solution incorporating elements of cable stay bridge technology consisting of long deviated tendons inside the spine beam box and short straight tendons (mostly) externally.

The straight tendons are hot-dipped galvanised, waxed, protected by medium density polyethylene sheaths and held in place by ultra-high performance fibre reinforced concrete anchors, known as 'blisters’. The 192 anchors have a maximum concrete strength of 170MPa, measure up to 1.5m by 900mm, and are shaped to fit the curved deck box. Each precast blister is connected to a 100mm thick internal backing slab, cast in situ through 12 core holes using a syringe.

The deviated tendons, up to 620m long, are unsheathed galvanised strands laid inside wax-filled high density polyethylene ducts. They have more-conventional anchors at the end of the flyover and in the soffit, constructed in self-compacting concrete. Local hydro-demolition was required to create suitable bedding for fixing the anchors.

Midspan short tendons (external) have 22 steel strands, each stressed to 21 tonnes, or 460 tonnes per tendon. Pier-capping short tendons (external and internal) comprise 13 steel strands and each have a total stress of 270 tonnes. The long tendons each contain 37 steel strands 16mm in diameter and are stressed to 770 tonnes, or 21 tonnes per strand.

The new post-tensioning increased the structure’s overall stress, resulting in the shortening, by up to 150mm, of the sections of flyover on either side of the single expansion joint. The existing joint and the two roller bearings beneath each pier did not have sufficient capacity to accommodate this. The structure is built-in at the abutments, and this was not changed.

The expansion joint was replaced with a comb joint. However, roller bearings similar to the originals were no longer available so sliding spherical bearings were fitted instead. To accommodate the larger heavier new bearings, up to 1.6m long and 2.3 tonnes in weight, each pier had to be jacked up using four 800 tonne jacks, requiring the bearing pits and foundation slabs had to be strengthened first. During a jacking operation, the flanking piers were also jacked slightly to relieve asymmetric loading.

The deck surfacing, waterproofing and central reserve have also been replaced. Drainage was kept above the waterproof membrane and improved with new drains in the central reserve and kerbs, and bespoke collection hoppers below deck.